The present invention relates generally to a speed control system for use in a motor vehicle, and more particularly to a speed control system which controls the intake air amount introduced into an engine of the motor vehicle in order to drive the motor vehicle at a target vehicle speed.
Generally, conventional speed control systems are based upon so-called proportion control in which an actuator is controlled in accordance with the results of comparison between a voltage with ripple representing an actual speed and a given voltage representing a target speed. However, in such a proportion control system, a steady-state deviation is necessarily present, thereby resulting in great speed variation. Therefore, in order to improve accuracy of control, it is generally known to incorporate an integration element into the control system. Such a speed control system for motor vehicles generally comprises an actuator for actuating a member provided for regulating the intake air amount introduced into an engine of the vehicle, a speed detector for detecting the speed of the vehicle, a deviation integrator for integrating the deviation of a vehicle speed obtained by the vehicle speed detector from a set target vehicle speed, and a drive signal generator for generating an actuator control signal on the basis of at least the integration value of the deviation obtained by the deviation integrator.
The deviation integrator continuously integrates the deviation of the detected vehicle speed from the target vehicle speed during the accelerating operation by a driver, and in response to the release from the accelerating operation the drive signal generator generates an actuator control signal corresponding to an integration value obtained at the time of the release, thereby regulating the intake air amount to be introduced into a vehicle engine. However, such the conventional speed control system suffers from taking a longer time for the vehicle speed to regain the target speed after the release from the accelerating operation and is subjected to a greater reduction of the vehicle speed during the regain interval.
In other words, when acceleration is performed with the vehicle driver depressing the accelerator pedal during speed control, the throttle valve is opened by the vehicle drive irrespective of a duty signal outputted from the control circuit to the actuator. During this, the deviation integrator continuously integrates the deviation. Therefore, there is a problem when, after stopping the acceleration by the vehicle driver, the duty signal whereby the actuator is controlled so as to drive the throttle valve to the closed direction is transiently outputted from the control circuit by the output of the deviation integrator accumulates abnormally, i.e., it is influenced by the integration value, irrespective of whether the actual vehicle speed goes below the target vehicle speed. After releasing from the constant-speed running state, it takes a long time until the actual speed follows the target speed, and undesired vehicle speed lowering can occur until the vehicle speed is coincident with the target speed.